In mammals, antigen-presenting cells (APCs) process foreign antigens. The processing of the antigen within the APC triggers an efficient immune response within the host. Antigens are degraded into peptide fragments and become bound to major histocompatibility complex (MHC) molecules that are expressed on the cell surface and are able to interact with other cells of the immune system. Dendritic cells, macrophages, and Kupffer cells in the liver are among the most commonly encountered type of APCs. These cells readily engulf foreign particles and express MHC molecules on their plasma membrane surface. These MHC surface molecules, know as human leukocyte antigens (HLA) in humans, are involved in the immune response against disease.
There are two classes of MHC molecules (class I and class II). MHC class I molecules display exogenous antigens (i.e., antigens taken up from outside the cell), whereas MHC class II molecules display endogenous antigens (i.e., antigens that originate within the cell) to the immune system. Processing of the antigens differ in each case. Processing of exogenous antigens by APCs occurs in stages. The cells first take up the antigens by endocytosis. The internalized endocytic vesicles then fuse with lysosomes where the foreign antigens are hydrolyzed by lysosomal enzymes, resulting in peptide fragments of 10 to 20 amino acids. These peptide fragments bind to a cleft within the MHC molecule and are transported to the cell's surface for interactions with cells of the immune system. The processing that leads to the display of endogenous antigens can arise from viral infection, etc. Such antigens are cleaved in the cellular cytosol, and transported into the lumen of the endothelial reticulum, where they become associated with MHC class I molecules, which are then transported to the cell surface. Foreign antigens can promote an immune response, whereas peptide fragments derived from cellular proteins when bound to MHC class I molecules and presented on the cell's surface are recognized as “self” and will not usually elicit an immune response.
Processed antigens displayed on self-MHC molecules supply one of the two signals required for T-lymphocyte activation. In addition to recognition of a foreign antigen fragment, simultaneous delivery of a co-stimulatory signal is needed for the activation of naïve T-lymphocytes. These co-stimulatory signals together with both class I and class II MHC molecules are molecules present on professional APCs. The presence of these molecules on professional APCs stimulate the clonal expansion of naïve T-lymphocytes, resulting in their differentiation into armed immune effector cells, and ultimately memory cells. Priming is a process where naïve T-lymphocytes are activated by the first time exposure to an antigen, whereas re-exposure to foreign antigens result in activation of memory cells.
Activation occurs when the T-lymphocyte's T cell receptor (an antigen-specific receptor) and its co-receptors (either CD4 or CD8 molecules) recognize the foreign peptide-MHC complex, simultaneously with a co-stimulatory signal delivered from the same APC. The best-characterized co-stimulatory molecules on APCs are CD80 (B7-1) and CD86 (B7-2). These structurally related molecules are members of the immunoglobulin superfamily and recognize the CD28 molecule on T-lymphocytes, resulting in T-lymphocyte activation. Activation of T-lymphocytes is controlled by the subsequent expression of CTLA-4. The CTLA-4 receptor is closely related to the CD28 molecule that binds the B7 molecules with a higher affinity than CD28 and prevents further T-lymphocyte activation.
There are numerous examples of how the addition of co-stimulatory molecules to cells affects cellular processes. In HIV/AIDS research: When CD3 and CD28 receptors on cultured T-lymphocytes are stimulated by immobilized monoclonal antibodies (mAbs), expansion of polyclonal CD4 positive T-lymphocytes occur. Ifthe T-lymphocytes were obtained from FIV-infected donors, HIV-1 viral load declines (in the absence of antiretroviral agents) simultaneously with T-lymphocyte expansion. Moreover, CD28 stimulation rendered these cells highly resistant to HIV-1 infection, mediating an antiviral effect early in the viral life cycle before HIV-1 DNA integration. The HIV-1 resistant state is specific for the macrophage-tropic HIV-1 isolates and is due to the lack of CCR5 receptor transcription, which is a required secondary receptor for HIV-1 macrophage-tropic virus infection. In tumor biology: The introduction of either MHC class II molecules or co-stimulatory molecules to tumor cells results in their efficient rejection in vivo. In virology: Viral infection with a number of the herpesviruses causes a diminution of cell surface co-stimulatory molecule expression, resulting in viral replication without mounting any immune response towards the infected cells.
Many viruses produce degenerative changes in cells when replicating in a susceptible cell culture. These characteristic changes are called cytopathic effects and are associated with certain morphologic changes in the host cell. The intracellular sites where the events of viral replication take place vary among the viral families. Enveloped viruses mature by a budding process, although some budding occurs with non-envelope containing viruses. For envelope viruses, viral-specific envelope glycoproteins are inserted into cellular membrances and the viral nucleocapsids then bud through the membrane at these modified sites. In this process, the virus acquires their envelope for infectivity and can also acquire cellular-related molecules. Studies with HIV, Influenza, and Chlamydia have shown virus particles that have incorporated HLA molecules into the mature virus particle. During the infection the cell is destroyed and the virus particles are released into the culture supernatant. The amount of infectious virus present in the cell culture fluid can be titrated and infectivity inactivated by a variety of methods. Although inactivated virus particles have lost the ability to replicate they maintain their structure, as detailed in this application, they can be used as a scaffold to carry cell surface expressed molecules.